The invention relates to a method for open-end rotor spinning, wherein the fibers to be spun are conveyed via a fiber guide channel into the rotor, are collected in its rotor groove of the largest interior diameter, are tied while being twisted into the yarn end in the area of a so-called tie-in zone by means of the rotor rotation and are drawn off as finished yarn through a draw-off nozzle, which is arranged centered and substantially on one level with the rotor groove.
The development of rotor spinning goes back a very long time, wherein the industrial use of this method only started on a larger scale in the sixties. A multitude of inventions was created not only in peripheral areas, i.e. from the sliver feed, the opening up into individual fibers and feeding of the individual fibers to the spinning rotor, as well as the drawing-off and winding-up of the yarn, but also in the core area of yarn formation, i.e. inside the rotor, only a small portion of which has entered into the present-day, very efficient automatic rotor spinning machines, which produce a yarn of high quality.
All methods have essentially in common that fibers from a sliver, which have been opened into individual fibers by means of an opening cylinder, are conducted together by means of a vacuum air flow to the rotor and are conveyed against a circumferential wall by means of the air flow and/or centrifugal force. As a rule, the shape of the inner rotor wall permits the collection of these fibers by forming an almost closed fiber ring. These collected fibers are continuously tied-up into a yarn end, wherein the yarn performs a true twist with every revolution of the rotor. The yarn rotation wanders opposite the yarn draw-off direction from the draw-off nozzle in the direction toward the yarn collection and, by the twisting of the doubled yarn, makes its continuous spinning on the open yarn end possible. The area where this piecing of the fibers to the yarn end takes place, is located between the detachment point of the yarn being created from the rotor wall, and the transition from the twisted yarn into the untwisted small sliver. It is called the tie-in zone.
Normally a yarn end for a piecing, which is fed into the rotor by the draw-off nozzle, is taken along in the direction of the rotor rotation by the air flow formed by the rotor rotation, at the latest when reaching the rotor groove. This curvature of the yarn end in the direction of rotor rotation is then maintained during the entire spinning process.
As can be seen from JP-OS 49-54 639, a malfunction can be caused by intensive soiling in the rotor, large bundlings of fibers, or the loss of the vacuum supply. The flipping of the curvature of the yarn end caused by this is quite undesirable, as stated in this Japanese laid-open document, since the yarn created in the course of this is said to show considerable disadvantages in respect to strength and evenness in comparison with a yarn, whose yarn end is curved in the direction of the rotor rotation. To prevent this flipping of the curvature opposite the rotor rotation direction, it is proposed in JP-OS 49-54 639 to arrange appropriate yarn contact elements on the draw-off nozzle and the rotor bottom, which are intended to stabilize the desired direction of curvature.
Within the scope of the further developments of the open-end spinning methods it was possible to definitely improve the processes, so that it is normally possible to avoid large collections of fibers, soiling or the failure of a vacuum. Accordingly, modern open-end spinning machine in principle are operated without additional aids for maintaining the curvature of the yarn end in the direction of rotation of the rotor.
A rotor spinning arrangement is described in xe2x80x9cBreakspinningxe2x80x9d, report of the Shirley Institute, Manchester, England, 1968, pages 76 to 79, wherein a funnel-shaped false twist element is arranged inside of the actual spinning rotor, which itself has the shape of a pan. This false twist element extends directly up to the fiber collection surface of the rotor. The rotor and the false twist element are separately seated and can also be separately driven. This means that the false twist element can be arranged in a stationary manner, as well as being driven in the direction of the rotor rotation, or opposite the direction of rotor rotation. Openings are arranged in the area of the collecting surface, by means of which a suction flow is created because of the centrifugal force of the rotor rotation. The fibers are fed in the radial direction on the collecting surface, which has the approximate shape of a cylinder surface. The yarn is drawn off through the rotor shaft, i.e. at the location opposite the fiber feed-in.
As described there, the relative direction of rotation of the yarn leg can be changed in relation to the rotor rotation as a function of the direction of rotation of the false twist arrangement. It is stated in conclusion that this relative rotation direction of the yarn leg clearly affects the yarn quality. Thus, in the positive direction, i.e. with the yarn leg running faster than the rotor, the yarn quality is said to be better by approximately 18% than with the oppositely directed relative speed of the yarn leg in relation to the rotor rotation.
A problem, which reduces the employment options of the rotor yarn produced on modern open-end rotor spinning machines, which otherwise has very even and good physical textile properties, resides in the formation of cover yarn, the so-called xe2x80x9cbelly bandsxe2x80x9d, which are wound in alternating directions of rotation either loosely, but partially very tightly, around the yarn periphery. The yarn structure, or the fiber orientation and fiber stretching, suffers because of this, with the result that the range of application of open-end rotor yarns becomes limited.
It is therefore the object of the invention to propose a method which limits the creation of cover yarn at least noticeably.
In accordance with the invention, this object is attained by providing a method for open-end rotor spinning, wherein the fibers to be spun are conveyed via a fiber guide channel into the rotor, are collected in its rotor groove of the largest interior diameter, are tied while being twisted into the yarn end in the area of a so-called tie-in zone by means of the rotor rotation and are drawn off as finished yarn through a draw-off nozzle which is arranged centered and substantially on one level with the rotor groove. The fiber flow exiting from a fiber guide channel has a directional component in the direction of rotation of the rotor, and the yarn leg extending from the draw-off nozzle to the rotor groove is curved, at least in the vicinity of the rotor groove, opposite the direction of rotation of the rotor during the spinning process.
The invention is advantageously further developed in a preferred embodiment of the method wherein the fiber flow is essentially fed to a fiber slide surface located between the rotor opening and the rotor groove. The direction of curvature of the yarn leg is created during the piecing process. In a first phase of the piecing process, a rotary flow directed tangentially opposite the direction of rotation of the rotor during its operation is caused to act on the yarn end introduced into the rotor for piecing, which flow is sufficient for creating the intended direction of curvature of the yarn leg. In such first phase of the piecing process, the rotor is initially driven opposite the direction of rotation of the rotor during its operation in such a way that the intended direction of curvature of the yarn leg occurs, and that the direction of rotation of the rotor during its operation does not exceed an angular acceleration which could lead to the flipping of the direction of curvature.
The method in accordance with the invention is based on the knowledge that, with a curvature direction of the yarn end in the direction of rotation of the rotor, fibers which, coming from the fiber slide face, directly reach the tie-in zone of the yarn end are initially tied to the twisting yarn in a direction which is opposite the normal yarn twisting direction, wherein in the course of the continued draw-off of the yarn, along with a simultaneous twisting thereof around its own axis, the direction of twisting of this fiber changes to the main yarn twisting direction. In those cases in particular in which the fiber reaches the tie-in zone first with its end located at the front in the direction of rotation of the rotor, several locally concentrated wraps can be created when the direction of rotation is changed. The yarn is constricted at this point with the result, that the yarn is uneven and the twist propagation is braked, which results in a loss of strength of the yarn in turn.
The setting in accordance with the invention of the curvature of the yarn opposite the direction of rotation of the rotor results in single fibers, which reach the yarn end in the tie-in zone, are immediately tied on, or in, in the normal twisting direction of the yarn and therefore do not cause any interference with the yarn production, nor a lack of quality arising therefrom.
Because of the detachment of the yarn end from the rotor groove, the angular speed of the detachment point, or of the tie-in zone, differs from the angular speed of the rotor. In the case of a curvature of the yarn end in the direction of rotation of the rotor, the angular speed of the tie-in zone is greater than that of the rotor, the tie-in zone runs ahead of the rotor. In the case of the present invention, with a curvature of the yarn end opposite the direction of rotation of the rotor, the tie-in zone trails behind the rotor. Because of this trailing of the tie-in zone, the fibers are drawn out of the rotor groove under an increased tensile stress. This results in additional stretching, which leads to an improved orientation of the fibers and makes possible an increased use of the strength of the fiber substance. In contrast to yarn which was produced with a leading tie-in zone, yarn produced in this way has a distinctive yarn core of stretched fibers.
The fact that with a trailing tie-in zone the fibers are tied to the yarn end with the same orientation with which they were conducted through the fiber guide channel to the rotor, also has an advantageous effect on the yarn structure. Here, the tangential alignment of the fiber flow in the direction of rotation of the rotor also assures the stretching of the fibers, because the inner surface of the rotor, i.e. the fiber slide surface, has a greater speed than the fiber flow impinging on it. This continuous maintenance of the stretching direction additionally furthers the stretched deposition of the fibers in the yarn structure.
By feeding the fiber flow onto a fiber slide surface, the fiber flow exiting the fiber guide channel is prevented from directly hitting the tie-in zone or the yarn end.
In accordance with the invention it is necessary to establish the trailing of the tie-in zone already in the course of the piecing procedure, in particular for obtaining an even yarn quality during the entire spinning process.
If no appropriate precautions are taken during the piecing procedure, leading of the tie-in zone automatically occurs because of the air flow which rotates along with the rotor. This orientation of the yarn leg is additionally aided by the rotation flow being created because of the tangential junction of the fiber guide channel and of the vacuum prevailing in the rotor housing. In the course of introducing the yarn end it is accordingly necessary to see to it that an opposite curvature is being formed.
This can be accomplished for one by generating a rotary flow opposite the direction of rotation of the rotor while the rotor still stands still, or does not yet rotate very fast, which acts on the yarn end being conducted from the draw-off nozzle to the rotor groove and which impresses the desired curvature on the yarn end. During this time the suction of the rotor housing can be maintained, since it aids the active air supply in the direction opposite to the rotation, which is in contrast to the passive suction of the fiber guide channel.
After the yarn end with the direction of curvature opposite the direction of rotation of the rotor has reached the rotor groove, this state is stabilized with the increasing number of rotor revolutions and therefore also the centrifugal force, and then remains as stable as in the state with a leading tie-in zone. In this connection the fact should be taken into consideration that the interferences mentioned in the prior art, which could cause a curvature change, are no longer relevant because of the command of the spinning process, as well as because of keeping the rotor clean.
The means used for generating the rotary flow can also be used for the so-called rotor flushing if it is necessary to remove the fibers which have reached the rotor in the course of a so-called fiber tuft equalization prior to the actual piecing (for example, see DE 197 09 747 A1).
Alternatively there is also the possibility of turning the rotor opposite its normal direction of rotation prior to the piecing procedure in order to cause in this way a deposition of the yarn end in this direction of rotation, which is opposite to the direction of rotation during its operation. In this case the suction of the rotor housing should be switched off in order not to endanger the desired deposition of the yarn leg by the suction flow, which causes a rotary flow in the direction of rotation of the rotor because of the tangential orientation of the fiber guide channel.
Following this, the rotor should be switched into the operating direction of rotation, but this process must not take place so abruptly that the direction of curvature of the yarn end flips again. Here, too, a stable curvature of the yarn end opposite the direction of rotation of the rotor is assured after the rotor has been run up. In addition, a slight twisting open of the yarn end during the rotation of the rotor opposite the normal operating direction also is advantageous for the piecing process. This yarn end, which has been opened further, is then better suited for a piecing process.
Besides the variations for creating the direction of curvature of the yarn end opposite the direction of rotation of the rotor described up to now, there are alternatively options of forming a fiber ring prior to introducing the yarn end into the rotor, or to switch the fiber flow into full strength after the yarn end has reach the rotor groove and the rotor has the number of rotor revolutions necessary for the process.
A further possibility for achieving the curvature in accordance with the invention of the yarn leg, or of the trailing thereof, consists in generating a yarn loop during the piecing procedure. In the course of this the yarn end is conveyed in the customary manner through the yarn draw-off tube into the rotor. Thereafter, a suction flow is generated in a radially spaced apart suction channel, while the spinning vacuum is shut off. Because of this the yarn end wanders from the draw-off nozzle into this suction channel. The feed length is regulated by the controlled feeding of the yarn through the yarn draw-off tube. At the end of feeding, the yarn end is clamped in the suction channel. Thereafter, a spinning vacuum is again generated and the rotor is started. Because of a continued return feed of the yarn, a larger size loop is formed between the draw-off tube and the suction channel. The air rotation caused by the rotor rotation pulls the loop in the direction of rotation of the rotor. After the loop has been sufficiently aligned in this way, the clamping is released, so that the yarn end can be deposited in the rotor groove opposite the direction of rotation of the rotor. Thereafter yarn draw-off is very rapidly accelerated, and the previously stepped yarn feed is restarted. In the process the yarn end is tied to the fibers. As in the already mentioned cases, the curvature of the yarn leg is stabilized by means of the centrifugal force then applied. With this process variation it is only necessary to see to it that no early feeding of fibers into the spinning rotor takes place in order to prevent the flipping of the yarn leg into the other direction of rotation in a phase which has not yet been stabilized by centrifugal force.
Stopping the yarn feed prior to the piecing process is not tied to a particular method here. For example, the fed-in fiber tuft can be deflected as long as is required by means of suction air directly downstream of the feed table. On the other hand, it is also possible to displace this point of the fiber flow deflection into the area of the fiber feed channel (for example, see DE 31 18 382 A1). It is only important that the fiber feed is completely stopped in the piecing phase in which the curvature of the yarn is formed.